The present invention relates to a technique effectively applied to a wave forming circuit that generates waveforms of any shape and frequency such as sine waves, triangular waves and trapezoidal waves, and more particularly to a technique effectively applied to a single-chip microcomputer having a tone signal wave forming circuit for communications control. Prior Art
Among the tone signal wave forming circuits that produce artificial sine waves of a particular frequency, there is, for instance, a tone generator S-7116A of Seiko Instruments Inc. make. This tone generator comprises such hardware as a program counter, a Johnson counter and a resistor ladder network, and outputs an artificial sine wave tone produced by a 5-bit digital/analog (D/A) convertor. The tone generator has six program signal input terminals and is capable of outputting artificial sinusoidal waveforms with 57 kinds of frequency in the range of 67 Hz to 2975 Hz.
This kind of tone generator is introduced by a book "ELECTRONIC COMPONENTS" published by Seiko Instruments Inc. in the section of Tone Generator S-7116A.
There is an invention (Japanese Patent Laid-Open No. 237248/1988), which discloses a pilot signal generating apparatus that comprises a read-only memory (ROM) for storing each voltage level of staircase waves and a counter's count value to set a timing for switching the voltage levels of the staircase waves, and that, based on the information stored in the ROM, outputs signals of different frequencies.
Japanese Patent Laid-Open No. 69904/1982 discloses a tone signal generating circuit, which produces staircase-like sine waves synchronized with clock signals. The amount of change in the voltage level that forms the staircase-like sine waves is variably controlled.
Japanese Patent Laid-Open No. 108218/1986 discloses a signal generating apparatus, which produces waveforms of any shape by adding a circuit to an existing resistor ladder .which comprises a series of equal-resistance resistors. The circuit controls such that the voltages at coupled points between resistors in the resistor ladder are nonlinearly distributed.
Japanese Patent Laid-Open No. 218208/1989 describes a technique for producing artificial sine waves by switching the coupled points of the resistor ladder connected across two power supply terminals according to the counter value.
Japanese Patent Laid-Open No. 238264/1989 describes a tone generation apparatus which comprises: a tone data output means for outputting a specified tone data; a tone attenuation processing means for providing tone data with a specified amount of attenuation; and an attenuation data changing means for changing a specified amount of attenuation periodically.
Japanese Patent Laid-Open No. 65059/1991 describes AC-DC power convertor equipment which successively switches a series of DC power supplies to produce staircase waveform signals simulating sine waveforms.